1. Field of the Invention
This invention relates to creating a read signal centerline estimate to provide a data-transition detection threshold for use with a pulse width modulated signal read from an optical recording. More particularly, the invention relates to tracking the read signal centerline estimate substantially near a mid-point between positive and negative peaks of the read signal. In addition, the invention relates to detecting defects in the optical recording and recovering the data-transition threshold after the defects.
2. Description of Related Art
Pulse width modulated (PWM) data recording is a recording method whereby a signal transition in a bit cell time interval indicates either a binary 1 state or a binary 0 state, whereas the lack of a signal transition in a bit cell time interval indicates the other of the two possible binary states 0 and 1. To decode the PWM read signal from a recording, the read signal must be assigned into bit cell time intervals, and the presence or absence of a signal transition, i.e. data transition, in a bit cell must be detected. The above-identified, cross-referenced application is directed to defining the bit cell time intervals and assigning the read signal transitions to the correct bit cell. The present invention is directed to keeping the data-transition detection threshold within a range for reliable use in detecting read signal data transitions and for making possible for the cross-referenced invention to reliably assign those transitions to the correct bit cell.
The transitions are defined as positive transitions (PTR), where the read signal goes from a low level to a high level, or negative transitions (NTR), where the read signal goes from a high level to a low level. Optimum detection of a transition is accomplished by providing a centerline threshold midway between the low level and the high level of the read signal. Each passage of the read signal through the centerline threshold then defines a transition, PTR or NTR, which generally defines a binary 1 for the bit cell containing the transition.
Data transition detection in PWM optical recording has the great advantage of providing a 6 db improvement in the signal-to-noise ratio over peak detection in PWM optical recording. The problems with PWM recording are the necessity of resynchronizing a clock signal to define the bit cells and the problem of the data-transition threshold drifting away from signal peak-to-peak centerline when there is a defect in the recording media.
A recording media defect might be caused by delamination in the recording medium, an irregularity in the recording surface or dirt on the recording surface. In any case, the result is that the read signal stays at a high or low level for an abnormal period of time which causes the data-transition threshold to drift away from a true centerline halfway between normal positive and negative peaks of the read signal. In some cases, the centerline estimate circuit operating with a data detection circuit may even lock the data-transition threshold onto a false centerline estimate above or below the true centerline.
The first problem of resynchronization is solved by the above-identified cross-referenced application. The second problem of maintaining the data-transition threshold at the true centerline between normal positive and negative peaks of the read signal is solved by the present invention.